During operation, automotive or aircraft turbocharger housings are subjected to elevated operating temperatures. These housings must be able to contain a turbine wheel generating very high rotational speeds. Exhaust gas from the automotive or aircraft engine initially contacts the turbocharger in metal sections, such as the gas inlet area of the turbocharger, at elevated temperatures. As high-speed performance improves through exhaust temperature increase, there have been attempts to gradually raise the exhaust temperature of the engine. Due to these high temperatures, the thermal load on the parts such as the exhaust manifold and the turbine housing becomes very great.
Various problems have been encountered by these increased exhaust gas temperatures contacting metal sections of the turbocharger. For example, one problem caused by the exhaust temperature rise is the problem of thermal deformation of the material, wherein the turbine housing and exhaust manifold, which alternates between regions of high temperature and low temperature is accompanied by thermal expansion and thermal shrinkage depending on the situation, which can cause surface oxidation wrinkles by such thermal deformation, and which can progress and develop into a penetration crack.
In order to overcome the challenges associated with higher operating temperatures, prior art alloys used in turbocharger applications have included alloys of higher nickel content such as commercially available high nickel ductile iron casting alloys. Examples of these are NiResist™ developed by the International Nickel Company, or HK30, a chromium-nickel-iron stainless steel alloy containing approximately 25% chromium and 20% nickel, with the balance essentially iron. The HK series stainless steel alloys in general have about 18-22% nickel and are fully austenitic.
The HK stainless steel alloys are strong stainless steel casting alloys, in terms of creep strength. However, while meeting the high temperature property requirements for turbocharger housings, they are quite expensive because of their high nickel content. Further, due to the sudden rise of the Ni price recently, there has been substantial concern over the cost of materials that have a high nickel content.
Accordingly, there is a need for stainless steel alloys useful in turbocharger applications that are able to withstand the higher operating temperatures produced by modern engines, but that minimize the expensive nickel content. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.